1. Field of the Invention
This invention relates to a polyester film containing a small amount of crosslinked polymer particles dispersed uniformly throughout.
More particularly, it relates to such polyester film containing fine powder (or fine particles or microparticles) of a crosslinked polymer having a particular sharp size distribution which is derived from a crosslinked polymer of specific porosity by pulverization.
2. Description of the Prior Art
In the present day, biaxially oriented films of polyesters, particularly of polyethylene terephthalate have found a wide applications in many technical fields, for example, preparation of gold and silver yarns, stamping foil, printing plate making, reprographics, releasing, photography, packaging and the like as well as electrical applications, for example, as an electrical insulating material, a capacitor dielectrode, a magnetic recording medium and the like, because of their superiority in various properties such as tensile strength, tear strength, modulus of elasticity, transparency, chemical resistance, thermal resistance, etc.
Such polyester films, however, are required to possess a specific combination of properties which is different from one application to another. For example, a so-called translucent polyester film useful for preparation of gold and silver yarns, stamping foil, plate making, releasing and the like is desired to have an excellent workability in handling of the film and to be unsusceptible to impairment of transparency.
Similarly, a polyester film for use in magnetic tapes as audio, video or computer magnetic recording medium is required to have a low coefficient of friction and a high abrasion resistance and to be unsusceptible to degradation of electro-magnetic transducing properties.
For use as a capacitor dielectrode, a polyester film is desired to be superior in workability and electrical characteristics.
As an attempt to improve these properties of polyester films, it has been known that fine particles of an inert material are included in the films by either of the following methods:
(i) Generally the residue of a metallic compound used as an ester exchange catalyst is caused by some means to precipitate as finely divided particles in the system (referred to as "precipitation method"). PA1 (ii) Microparticles of an inorganic compound with varying particle size are added from the outside (referred to as "addition method").
These methods, however, suffer from some disadvantages as described below.
According to Method (i), since the amount and diameter of particles precipitated are variable, the slip property of the resulting film is difficult to control and is insufficient considering its high film haze. Another disadvantage is that the original slip property cannot be reproduced after regeneration of the film for reuse.
On the other hand, Method (ii) requires the procedures of pulverization and classification of an inorganic compounds. In addition, an inorganic compound generally has a poor compatibility with the polyester matrix of an organic nature even if the former had been subjected to the above mentioned procedures. Therefore, microparticles of the inorganic compound are difficult to disperse uniformly in polyester and are frequently agglomerated into coarse particles therein. The presence of foreign materials other than fine particles or of coarse particles or badly-dispersed agglomerates of inorganic compounds in a polyester film impairs the film properties, for example, in a film for use in capacitors, by adversely affecting the electrical properties of the film, and, in a film for use in magnetic tapes, by degrading the electro-magnetic transducing characteristics or by causing formation of white dust or drop out.
Therefore, there is a continuing need for an improved polyester film.